The present invention relates to an injection device of the type that receives a syringe, extends it, discharges its contents and then retracts it automatically. Devices of this general description are shown in WO 95/35126 and EP-A-0 516 473 and tend to employ a drive spring and some form of release mechanism that releases the syringe from the influence of the drive spring once its contents are supposed to have been discharged, to allow it to be retracted by a return spring.
Often, such injection devices are required to work with glass pre-filled syringes that were originally designed for manual use. Such glass syringes have a flange at their base to allow a user to grip the syringe. The substantial force produced by the drive spring is applied to the piston of the syringe. This force is transferred to the housing and return spring, via the flange. The flanges are not precision molded and consequently have low manufacturing tolerances. They are not sufficiently flat or consistent to be used as a satisfactory support means for the syringe through which the force of the drive spring is transferred to the housing and return spring.
In practice, these flanges have been seen to fail when the drive spring is employed and the force produced by the drive spring is applied, via the piston of the syringe, to the flange. In particular, these flanges have been seen to break off from the syringe, resulting in the syringe body being propelled from the front of the injection device, and the whole needle being inserted into the user's body. Consequently, when the injection device is taken away from the user's body, a full, broken syringe is left dangling from the user's body. This is clearly dangerous because the user is left with a broken syringe, and consequently broken glass, dangling from their body. The user is also left without having had their correct dose of drug. Such a syringe failure is also, of course, unpleasant for any user, particularly those that are squeamish.